The present invention relates to exposure apparatuses and device fabrication methods using the same. The present invention is particularly suitable for an exposure apparatus used for micro-lithography to form a fine pattern for semiconductors, liquid crystal devices (“LCDs”), magnetic materials, etc.
The device fabrication using the lithography technique has employed a projection exposure apparatus that uses a projection optical system to project a circuit pattern formed on a mask or reticle (these terms are used interchangeably in this application) onto a wafer, thereby transferring the circuit pattern.
In general, the projection exposure apparatus includes an illumination optical system for illuminating the mask using light emitted from a light source, and the projection optical system, located between the mask and an object to be exposed. The illumination optical system typically introduces light from the light source to an optical integrator, such as a fly-eye lens, to obtain a uniform illumination area, and uses an optical-integrator exit surface as a secondary light source surface to Koehler-illuminate a mask surface via a condenser lens.
A most effective light source should be formed according to reticle patterns for high quality exposure. The effective light source means an angular distribution of the exposure light incident onto a wafer surface. For example, this effective light source distribution is implemented by adjusting to a desired shape a light intensity distribution near the fly-eye-lens exit surface i.e., a secondary light source surface.
FIG. 9 shows a relationship among a secondary light source distribution, a pupil transmittance distribution, and an effective light source distribution in a conventional exposure apparatus. Although the secondary light source distribution may form various shapes including a circular shape and an annular shape according to reticle patterns, it indicates an illumination condition with coherence factor σ=0.8. As in the illustrated secondary light source distribution, the conventional light intensity distribution has been adjusted to be uniform or flat. Since the pupil transmittance distribution is approximately uniform, the effective light source distribution becomes uniform on the wafer surface, providing σ=0.8 uniformly without any difference of effective light source distribution between an on-axis and an off-axis.
The resolution R of the projection exposure apparatus is given by the following equation where λ is a wavelength of the light source, NA is the numerical aperture, and k1 is a constant determined by a development process and others:R=k1(λ/NA)  (1)
The recent demands for highly integrated devices have increasingly required fine patterns to be transferred or higher resolution. From the above equation, a higher numerical aperture NA and reduced wavelength λ would be effective to obtain the higher resolution.
Thus, an exposure light source used for the exposure apparatus has shifted to a shorter wavelength from i-line (with a wavelength of 365 nm) to KrF excimer laser (with a wavelength of 248 nm), ArF excimer laser (with a wavelength of 193 nm), and even F2 laser (with a wavelength of 157 nm). NA has shifted larger from 0.7 to 0.75.
However, such a short wavelength as 200 nm or less and NA of 0.70 or higher (i.e., high NA) have created a problem in that the secondary light source distribution and the effective light source distribution do not accord with each other, and the effective light source distribution does not become uniform even when the secondary light source distribution is made uniform, lowering the exposure performance.
In other words, the higher NA results in a larger light incident angle onto each optical element, making it difficult to maintain constant an angular characteristic of the transmittance (and reflectance) in a required incident angle area. More specifically, the transmittance near the optical axis appears to be low because a light transmitting element, such as a lens, is thick at its center part and thin at its peripheral in view of the glass material's transmittance, but the transmittance at the peripheral actually becomes lower because a coating or reflection prevention film affects the transmittance more greatly. This is because the transmittance decreases more remarkably as a refraction angle of light incident onto the optical element becomes larger due to the coating, and the light transmitting through the peripheral of the light transmitting element has larger refraction angle than that transmitting through its center part. Although the conventional design technique has succeeded in maintaining within a permissible range, the transmittance reduction caused by the refraction angle at the peripheral, the refraction angle has become larger with the higher NA and the transmittance reduction has been unable to be maintained within the permissible range. In addition, the shorter wavelength limits usable materials for a reflection prevention film applied onto the transmission member, and restricts a degree of freedom of design.
FIG. 10 shows a relationship among a secondary light source distribution, a pupil transmittance distribution, and an effective light source distribution in an exposure apparatus having a high NA. The secondary light source distribution is set to have a uniform coherence factor a σ=0.8, similar to FIG. 9. However, the lower pupil transmittance distribution at its peripheral as shown in the middle in FIG. 10 results in a non-uniform effective light source distribution, as well as a non-uniform effective σ value of less of less than 0.8.
FIG. 11 shows a relationship among a secondary light source distribution, a pupil transmittance distribution, and an effective light source distribution in an exposure apparatus having a catadioptric projection optical system. The secondary light source distribution is set to have a uniform coherence factor σ=0.8, similar to FIG. 9. However, the non-uniform pupil transmittance distribution as shown in the middle in FIG. 11 prevents from the effective light source distribution from being uniform, as well as providing a non-uniform effective σ value of less of less than 0.8. In particular, a mirror has different reflectance depending upon a deflection angle. Here, the pupil transmittance in this application means optical use efficiency in an optical system including reflectance.
Thus, the optical system in the exposure apparatus has different transmittances between part near the optical axis and part apart from the optical axis, and provides eccentric angular distribution of the exposure light incident onto the wafer (i.e., effective light source distribution). Disadvantageously, this results in an undesirable angular distribution of the exposure light incident onto the wafer surface (i.e., effective light source distribution) even when the secondary light source distribution is adjusted to a desired distribution since the subsequent optical system has a non-uniform transmittance distribution (or pupil transmittance distribution). In other words, a predetermined resolution critical dimension (in particular, minimum line width) cannot disadvantageously obtained because this results in exposure with a coherence factor different from a most suitable one.
This problem occurs at wafer center positions (on-axis) and wafer peripheral (off-axis), but another problem of an offset center of gravity occurs at the off-axis in which a deviation of an effective light source distribution incident onto the on-axis deviates differently from that incident onto the off-axis. As a result, in addition to the above problem, the off-axis critical dimension to be transferred to the wafer is different according to positions.